In a positive electrode active material used for nonaqueous electrolyte secondary batteries, a phenomenon (cracks in particles) sometimes occurs in which cracks are generated in an active material particle while the expansion and contraction of the particle are repeated associated with charge/discharge cycles. The cracks in particles, when generated, become a cause for bringing about deterioration in performance as a nonaqueous electrolyte secondary battery, such as an increase in internal resistance.
Attempts have been made to enhance compressive breaking strength of a particle for suppressing the cracks in the particle caused by the expansion and contraction. Specifically, the hardness of a particle is enhanced by increasing the size of a crystallite. It is described in Patent Document 1 that the collapse of a crystal structure due to the movement of a Li ion associated with charge/discharge can be suppressed to the minimum by: using a positive electrode active material comprising a particle formed by an aggregate of fine monocrystals called crystallites; making the shapes of the crystallite and the particle almost sterically isotropic; and making the crystallite diameter in the (003) vector direction of the crystallite from 500 to 750 angstroms. Moreover, it is disclosed in Patent Document 2 that an increase in filling amount due to sliding among particles can be achieved and a positive electrode having a high capacity can be obtained by making the compressive breaking strength of the particle high.